In an image-forming device, sometimes the positions of images formed on the recording medium can become shifted from their correct positions when the body of the image-forming device receives an impact, for example. For this reason, some conventional image-forming devices have been provided with a function to correct offset in the image-forming positions. This type of image-forming device transfers marks formed on photosensitive members for detecting registration error onto a conveying belt that is driven to convey the recording medium, detects the positions of the marks with a photosensor or the like, and corrects the positions of images formed on the photosensitive members based on the detection results. Accordingly, the image-forming device can form high-quality images with a reduction in registration error.
Japanese unexamined patent application publication No. HEI-9-193476 describes an image-forming device that forms a plurality of marks for detecting registration error, each mark configured of a set of four colors, where the writing positions of the marks are set to positions of opposite phase relative to the rotational period of the photosensitive member. This construction is designed to prevent dynamic positional offset caused by rotational irregularities of the photosensitive member from adversely affecting the accuracy in correcting registration error.